Impact of Submarine Groundwater Discharge on Marine Water Quality and Reef Biota of Maui

Abstract

Generally unseen and infrequently measured, submarine groundwater discharge (SGD) can transport potentially large loads of nutrients and other land-based contaminants to coastal ecosystems. To examine this linkage we employed algal bioassays, benthic community analysis, and geochemical methods to examine water quality and community parameters of nearshore reefs adjacent to a variety of potential, land-based nutrient sources on Maui. Three common reef algae, Acanthophora spicifera, Hypnea musciformis, and Ulva spp. were collected and/or deployed at six locations with SGD. Algal tissue nitrogen (N) parameters (δ15N, N %, and C:N) were compared with nutrient and δ15N-nitrate values of coastal groundwater and nearshore surface water at all locations. Benthic community composition was estimated for ten 10-m transects per location. Reefs adjacent to sugarcane farms had the greatest abundance of macroalgae, low species diversity, and the highest concentrations of N in algal tissues, coastal groundwater, and marine surface waters compared to locations with low anthropogenic impact. Based on δ15N values of algal tissues, we estimate ca. 0.31 km2 of Kahului Bay is impacted by effluent injected underground at the Kahului Wastewater Reclamation Facility (WRF); this region is barren of corals and almost entirely dominated by colonial zoanthids. Significant correlations among parameters of algal tissue N with adjacent surface and coastal groundwater N indicate that these bioassays provided a useful measure of nutrient source and loading. A conceptual model that uses Ulva spp. tissue δ15N and N % to identify potential N source(s) and relative N loading is proposed for Hawaiʻi. These results indicate that SGD can be a significant transport pathway for land-based nutrients with important biogeochemical and ecological implications in tropical, oceanic islands.

Carbonate system parameters of an algal-dominated reef along West Maui

“…Despite being situated in an oligotrophic region with naturally occurring, low nutrient concentrations, anthropogenic nutrient loading to coastal waters via sustained SGD is driving nearshore eutrophication (Dailer et al, 2010Bishop et al, 2015;Amato et al, 2016;Fackrell et al, 2016), with algal δ 15 N signatures at Kahekili Beach Park indicative of wastewater effluent (Dailer et al, 2010. In response, there has been a shift in benthic cover from abundant corals to turf or macro-algae over the last two decades.…”

“…Higher islands, therefore, have the potential for not only greater orographic rainfall, and thus submarine groundwater recharge, but also greater potential pressure head and thus enhanced SGD-driven nutrient fluxes. There is also greater potential for enriched nutrient sources and reduced water quality with fast-growing population and development (Amato et al, 2016;Fackrell et al, 2016). Thus, SGD represents a key vector of nutrient loading in tropical, oligotrophic regions (e.g., Paytan et al, 2006).…”

Nutrient dynamics in submarine groundwater discharge through a coral reef (western Lombok, Indonesia)

“…Different measuring approaches were used such as satellite thermal infrared images (Johnson et al ), naturals tracers (Peterson et al ; Knee et al ; Swarzenski et al ), salinity mass balances (Lee and Kim ), velocity and hydrography measurements (Valle‐Levinson et al ), or recharge estimates in the adjacent terrestrial watershed (Sawyer et al ). Significant inputs of inorganic nitrogen into coral reef ecosystems occur, for example, on Hawaii (Amato et al ; Swarzenski et al ) or Rarotonga, where SGD contributed to 81% of dissolved inorganic nitrogen inputs into a coral reef lagoon (Tait et al ).…”

Integration of aerial infrared thermography and in situ radon‐222 to investigate submarine groundwater discharge to Pearl Harbor, Hawaii, USA

“…Moreover, concentrations of dissolved constituents in SGD are often greater than in surface waters resulting in significant groundwater-derived solute contributions (e.g., Zektser et al 2006;Bishop et al 2017). Submarine groundwater discharge impacts nearshore ecosystems and influences nutrient balances, potentially causing eutrophication events, harmful algal blooms, and shifts in the dominant flora and fauna of coastal waters (e.g., Dollar and Atkinson 1992;Paerl 1997;Miller and Ullman 2004;Dailer et al 2012;Amato et al 2016).…”